The reduction in weight of component parts generally made of metals such as steel and aluminum has been an important goal of many industries. Reduction in the weight of automobiles, for example, has significant implications for energy use while reduction in the weight of many tools and consumer devices has potentially favorable ergonomic impacts. A key element in this effort to reduce weight is the need to maintain material stiffness in many applications. While much success has been achieved using filled polymers, and other composite materials, these are often expensive and difficult to form using traditional methods such as cold stamping. Recent developments in the field of metal-thermoplastic-metal laminate structures have been very encouraging. When appropriately processed, these materials can provide not only significant weight reduction, but also allow the needed material stiffness and formability. One such material is an aluminum-polypropylene-aluminum laminate manufactured by Hoogovens Hylite B.V. and named Hylite. This material is 60% lighter than steel and 30% lighter than aluminum while maintaining considerable stiffness, yield strength and tensile strength.
The ultimate usefulness of such lightweight laminates depends on the ability to form the material to useful shapes with reasonably efficient processes and with a precision and accuracy that reflects the needs of the design. It would be desirable to be able to use forming methods typical of those currently in use for metal panels such as die forming etc. Also critical to the use of the material is maintenance of the desired shape after processing. Common methods of forming aluminum and steel parts include stamping, dieing, and forging. A disadvantage of all of these methods is the high force required to overcome the yield strength of the metals. These high forces require very strong dies usually made out of thick and hardened steel. Such common, lower energy forming methods as thermoforming and vacuum forming which are commonly used with plastic materials are generally not considered for forming metals. In addition, when preparing panels and other surfaces where the radius of curvature is relatively large (greater than 3 inches) great difficulties can be found with "spring-back" of the metal such that design modifications to the forming tools are necessary to obtain the desired material shape. In these instances, small variations in material properties can also have profound influence on the dimensions of the final part.
Several methods for forming the metal-thermoplastic-metal laminates have been revealed in the art. While the conventional stamping and die forming methods used for metallic panel materials also work for the metal-thermoplastic-metal laminate, specialized forming processes have been developed to overcome some of the problems encountered with the laminate.
A method of hot-forming described in GB 1092715 consists of placing a thermoplastic sheet between two specially treated aluminum foils and heating the resulting sandwich in a die to coincidentally bond the laminate and form the shape in the same operation. After suitably heating, the die is cooled and the part removed. This method has several disadvantages. Where components for mass production are involved, the heating and cooling of the die creates unacceptably long cycle-times. In addition, the repeated heating and cooling of the forming tool over a broad temperature range (175.degree. C. to 95.degree. C.) for each part formed results in significant wasted energy. Finally, inventories of the specially treated aluminum panels and thermoplastic sheets must be specially managed and in the most desirable case where the aluminum sheets are very thin, great care taken during handling to avoid damage.
Solutions to several of the problems mentioned above are given in EP 547664 which describes preheating a pre-made laminate (e.g. Hylite) to a temperature just below the Vicat softening point of the thermoplastic. The preheated laminate is then formed by placing it in contact with a forming tool such as a matched metal die. While this method overcomes the problems with handling the thin metal foils and managing inventories of the laminate components, it still requires considerable pressure in the forming process and is susceptible to spring-back from the forming tool for large radius shapes. In addition, variations in the thermoplastic core of the laminate or in the mechanical properties of the metallic skins, continue to make the production of the desired shape difficult to accomplish with a high degree of precision.
A third technique, described in EP 598428 employs forming the laminate under ambient conditions followed by heating of the formed piece to relieve the stresses built up in the laminate during the forming process. This process provides the formed laminate with improved resistance to shape change on subsequent exposure to elevated temperature. The post heat treatment is done at near the softening point of the thermoplastic, but well below the melting temperature. While this method allows one to take advantage of existing equipment that might be on a conventional metal processing line, use of the initial ambient temperature forming step shares the same dimensional problems as mentioned above.
The processes of the art do not provide solutions to several key problems encountered in the forming of parts from the metal-thermoplastic-metal laminate. The need to design forming equipment to compensate for both spring-back and lot to lot material variations is the most serious of these. Not only do complex design parameters need to be considered to design the tools, but lot-to-lot variations in such properties as yield strength, make the production of parts which accurately reflect the designers intent very difficult. The processes of the art also continue to require significant mechanical energy in the forming step because of the stiffness of the panel materials, eliminating such efficient and energy efficient process such as thermo- and vacuum-forming from consideration.